Linear positive displacement flow meters typically utilize a piston inside a cylinder. The substance to be measured is directed into the cylinder and the flow of the substance causes the piston to be displaced along the cylinder. The piston displacement is measured, and since the piston area is known, the displacement can be converted mathematically to a flow rate. The flow of the substance being measured can be linear, or in the case of diesel injection systems, pulsating.
An “opposing force”, i.e. a force opposing the substance, is normally applied to the piston. The application of the opposing force has three main effects:                preventing the piston from overshooting at the end of the cylinder;        preventing oscillation of the piston, when a pulsating flow is being measured;        maintaining the substance being measured under compression, thereby preventing errors in reading due to air coming out of solution when the substance in a fluid;        returning the piston back towards its datum when full travel has been reached, or when the measured substance needs to be drained from the cylinder.        
For the substance to move the piston, the “substance force”, i.e. the force applied to the piston by the substance, must overcome the opposing force being applied to the piston. From actuation of the meter, the substance force increases, until it reaches a threshold point, at which it is equal to the opposing force. The point at which the threshold point is reached is indicated by ‘X’ on FIG. 1, which is a graphical illustration of substance force (Newtons), against number of injections of fuel from a diesel fuel injector. As the force applied by the substance increases above the threshold point, the piston will begin to move, against the opposing force.
Any flow rate measurements taken before the threshold point is reached will be incorrect and need to be discarded.
An increase in the volume or a reduction in the force on the measurement cylinder side of the piston will require measurements to be discarded until the substance force once again overcomes the opposing piston force. This typically occurs after the substance is emptied from the cylinder. A change in volume may also be caused by upstream hydraulic valves that divert the flow from multiple injectors into the metering unit. It may be worth mentioning this.
As illustrated in FIG. 1, substance force increases up until the sixteenth injection, at which point it reaches the threshold value. Therefore first 16 measurements taken by the meter are invalid and should be discarded.
Known linear positive displacement flow meters typically discard measurements following a reduction in substance force using a combination of the following:                a time duration;        a piston displacement;        a number of flow pulses.        
The algorithm used to discard invalid measurements is normally derived through experiment, over a range of flow conditions. However, the algorithm itself is susceptible to errors in test conditions, due to variations in system dynamics caused by, for example:                errors in the opposing force on piston;        viscosity and density of the substance;        temperature of the substance and meter;        humidity;        altitude;        friction;        wear of components.        